Service providers often deliver VLAN services to enable customers to set up a virtual LAN over the service provider's network fabric. The VPLS allows the connection of multiple customer premises equipment sites (CPEs) in a single bridged domain over a provider-managed IP/MPLS (multi-protocol layer switching) network. VPLS is a transparent, protocol-independent service, in which the CPEs in a VPLS instance appear to be on the same LAN regardless of their actual location. CPEs are connected to the IP/MPLS network through an access cloud, which is whatever communications fabric which lies between the CPEs and the edge of the IP/MPLS network. At the edge of the IP/MPLS network are provider edge nodes which form a major part in delivery of VPLS. In order to provide redundancy and protection from link failure, protection switching between the access cloud and the provider edge nodes is often employed. Single and multi-chassis APS (automatic protection switching) may be employed to help ensure delivery of service between the access cloud and provider edge nodes. Known methods of APS may be applied to VPLS to minimize traffic interruption and attempt to help VPLS reconvergence.
Referring now to FIG. 1, known delivery of VPLS is described.
In FIG. 1 first customer premises equipment 210 and second customer premises equipment (CPE2) 220 are connected to an access cloud 200 which lies outside of the VPLS core IP/MPLS network 400 of the service provider. Access cloud 200 has an access switch 202. The access cloud 200 and switch 202 may for example be ATM/SONET/SDH/frame relay, as long as access to the VLAN provider edge nodes of the VPLS service is provided. The access switch 202 is linked to a first provider edge node (PE1) 300 via an access link 280. Link 280 may be a single circuit or may be an APS 1+1 redundant pair in a multiport single node arrangement for example. The first provider edge node 300 of the VLAN is linked through the VPLS core network 400 to other provider edge nodes through tunnels. A first tunnel 290 links the first provider edge node 300 to a second provider edge node (PE2) 310. The second provider edge node 310 is linked via a second tunnel 292 to a third provider edge node (PE3) 320. The first provider edge node 300 and the third provider edge node 320 are linked by a third tunnel 294. Fourth customer premises equipment (CPE4) 250 and fifth customer premises equipment (CPE5) 260 are linked to the third provider edge node 320 and third customer premises equipment (CPE3) 240 is linked to the second provider edge node 310.
In the VPLS core 400, each provider edge node 300, 310, 320, possesses a MAC address table having MAC address entries which provide information used by the provider edge node 300, 310, 320 to link with each customer premises equipment 210, 220, 240, 250, 260. Each MAC address entry in the MAC address table of a particular provider edge node contains the MAC address of a customer premises equipment and a MAC address mapping value which designates the link to the CPE. When the customer premises equipment is remote from the particular provider edge node and accesses the VLAN through a different provider edge node, the MAC address mapping value designates the tunnel linking the particular provider edge node with the different provider edge node. Provider edge nodes learn MAC address mapping values with the customer traffic sent through access ports and over the IP/MPLS network. Customer traffic from an originating provider edge node having a destination customer premises equipment which has an unmapped MAC address at the originating provider edge node is broadcast to all other provider edge nodes 300, 310, 320 participating in delivery of the VPLS. The MAC address mapping value designating the tunnel which allows VPLS delivery to the destination customer premises equipment is learned by the originating provider edge node from a reply from the destination customer premises equipment, after which customer traffic is sent unicast towards the destination customer premises equipment with use of the MAC address mapping value. The MAC address mapping value of a MAC address entry corresponding to a customer premises equipment can also be learned by a provider edge node by receiving customer traffic from that customer premises equipment.
For example, if first customer premises equipment 210 were to try to access third customer premises equipment 240 in the VLAN, the first provider edge node 300 would broadcast a message from the first customer premises 210 to all provider edge nodes. Third customer premises equipment 240 would answer through PE2 310. After receiving a response from third customer premises equipment 240, first provider edge 300 would save a MAC address entry in the MAC address table having a mapping value designating the first tunnel 290 as the way to access third customer premises equipment 240 through PE2 310. Once first provider edge node 300 has the MAC address mapping value for third customer premises equipment 240 designating the first tunnel 290 in first provider edge node's 300 MAC address table, any traffic thereafter destined for third customer premises equipment 240 through first provider edge node 300, would be unicast through the first tunnel 290 thereby reducing network traffic on tunnels which are not needed and saving network resources. Concurrently, the second provider edge node 310 would save the MAC address mapping value designating the first tunnel 290 as the way to access first customer premises equipment 210 through the first provider edge node 300. For service between first customer premises equipment 210 and fifth customer premises equipment 260, the first provider edge node 300 would broadcast a message from the first customer premises equipment 210 to all other provider edge nodes to reach fifth customer premises equipment 260 which would answer through the third provider edge node 320. The first provider edge node 300 would learn the MAC address mapping value designating the third tunnel 294 as the way to access fifth customer premises equipment 260 through the third provider edge node 320, while concurrently the third provider edge node 320 would learn the MAC address mapping value designating the third tunnel 294 as the way to access first customer premises equipment 210 through the first provider edge 300. In general, provider edge nodes learn the proper tunnel to destination customer premises equipment from a response to a broadcast or by receiving traffic from the customer premises equipment. This tunnel to the destination is kept as a MAC address mapping value in its MAC address table, and will be used to unicast any further traffic directly to that destination customer premises equipment.
In providing communications services to customers, service providers attempt to ensure that services are delivered without loss of data and with minimal interruption. This applies especially to the links between the provider edge nodes on the edge of a VPLS network, and the access equipment immediately down/up stream of the provider edge node towards customer premises equipment. A well known approach to ensuring data transfer services is automatic protection switching or APS. In SONET/SDH, APS 1+1 is typically used for single chassis protection switching.
Referring now to FIG. 2A, known single chassis APS 1+1 is discussed. A near end (NE) chassis 10 having SONET line-terminating equipment (LTE) whose data traffic is to be protected, has an NE working port 21 which is linked via a bi-directional working link 14 to a far end (FE) working port 23 of an FE chassis 20 having SONET LTE. The NE chassis 10 is also linked from an NE protection port 25 over a bi-directional protection link 16 to an FE protection port 27 of the FE chassis 20. In this configuration, the NE chassis 10 is said to be protected by an APS group having a working circuit made up of the NE working port 21, the working link 14, and the FE working port 23, and having a protection circuit made up of the NE protection port 25, the protection link 16, and the FE protection port 27.
Typically the working circuit carries the data traffic which is to be protected. When a circuit is carrying the data traffic, it is said to be active, and when it is not carrying the traffic it is said to be inactive. For consistency the link and ports of an active circuit are referred to as being active, and the link and ports of an inactive circuit are referred to as being inactive. In automatic protection switching the working circuit is typically active when there is no failure.
In the event of a failure or degradation of the signal of the active circuit, which may be caused by failure or degradation of the active link or either active ports, APS 1+1 switches the data traffic from traversing the failed or degraded circuit to traversing the other circuit. The other circuit becomes active and the failed or degraded circuit becomes the inactive circuit. Since each single chassis has control of a working port and a protection port, it is not difficult to switch the data traffic from the working circuit to the protection circuit.
In the context of VPLS as illustrated in FIG. 1, the access link 280 could be an APS 1+1 single chassis redundant link between the access switch 202 and the first provider edge node 300. In other words the access switch 202 of FIG. 1 would operate as the near end chassis 10 of FIG. 2A, the access link 280 would be made up of a working link and a protection link similar to those 14, 16 depicted in FIG. 2A. Finally, the first provider edge node 300 of FIG. 1 would operate as the far end chassis 20 of FIG. 2A.
The APS 1+1 architecture also allows for the protection circuit and the working circuit to be configured to end at two different FE chassis. Such a known configuration protects against nodal or router failures in addition to link and circuit failures.
Referring to FIG. 2B, a known dual chassis APS 1+1 configuration is discussed. A near end (NE) chassis 110 having SONET line-terminating equipment (LTE) whose data traffic is to be protected, has an NE working port 153 which is linked via a bi-directional working link 114 to a far end (FE) working port 157 of a first FE chassis 120 (labeled “CHASSIS A” in FIG. 2B) having SONET LTE. The NE chassis 110 is also linked from an NE protection port 155 over a bi-directional protection link 116 to an FE protection port 159 of a second FE chassis 130 (labeled “CHASSIS B” in FIG. 2B) having SONET LTE. The first and second FE chassis 120, 130 are linked together via control link 140.
In this configuration, the NE chassis is protected by the APS group having a working circuit made up of the NE working port 153, the working link 114, and the FE working port 157, and having a protection circuit made up of the NE protection port 155, the protection link 116, and the FE protection port 159.
The second FE chassis 130 is referred to as the protection chassis or chassis in protection mode, and it is in constant communication with the first FE chassis 120 which is referred to as the working chassis or chassis in working mode.
In the event of a failure or degradation of the signal of the active circuit, which may be caused by failure or degradation of the active link, either active ports, or the working chassis, APS 1+1 switches the data traffic from traversing the failed or degraded circuit to traversing the other circuit. Since the FE chassis 120, 130 are remote from each other, FE chassis 120, 130 need to exchange switching control signals over the control link 140 to coordinate the switching from the working circuit to the protection circuit. FE chassis which exchange switching control signals over the control link 140 are said to be members of a redundant APS pair, each being an APS peer of the other within the pair. In switching the data traffic, the protection circuit becomes an active circuit and the failed or degraded working circuit becomes an inactive circuit.
In the context of VPLS, protection switching between the access switch 202 and the provider edge nodes 300, 310, 320, can be implemented using a multi-chassis APS configuration as discussed in association with FIG. 2B. In such a configuration, one provider edge node would act as a working chassis and another provider edge node would act as a protection chassis. After an APS switchover, however, other provider edge nodes will keep sending traffic to the de-activated provider edge node until they relearn the MAC address mappings of the customer premises equipment of the newly active provider edge node (previously the protection chassis). This causes black-holing of traffic for a time associated with layer 2 functionality such as hold times, time outs, or keep alive periods. This duration of black-holing is even worse if there is little or no traffic flowing from the new active provider edge node to the other provider edge nodes participating in the VPLS.
The manner in which the switching from the working circuit to the protection circuit is carried out and the particulars of how an APS configuration at the edge of a VPLS enabled network is used can have a very important effect on the resilience of the VPLS re-convergence and hence determine the duration and magnitude of the service interruption associated with black-holing.